The realization that humans are changing the Earth's climate is profound, and yet it is only one of many ways in which humans are changing the physical and biological environment. As discussed in Unit 1, "Many Planets, One Earth," the arrival of humans to the Americas at the end of the last ice age, approximately 14,000 years ago, was accompanied by the extinction of most large mammals, including mammoths and mastodons, presumably from excessive hunting. As discussed in Unit 9, "Biodiversity Decline," human land-use has caused an enormous reduction in biodiversity, as sensitive ecosystems such as wetlands, tropical rainforests, and coral reefs are encroached or destroyed by human activities. Confronted with these and other impacts of human activities, why is anthropogenic climate change so troubling? The answer is that climate change has the potential to make many of the other environmental challenges much more difficult to solve because of the global scale of the impacts and the huge magnitude of the change relative to what the Earth system has experienced over the last tens of millions of years.

Given the dramatic changes we are observing today in the climate system, coupled with our view from Earth history, what can be done? The first challenge we must confront in working toward a solution to future climate change is that any "solution" will be incomplete. Some amount, perhaps even a substantial amount, of climate change is unavoidable. Reducing CO2 emissions so that they are below the level of CO2 uptake by the oceans and biosphere will not happen in a decade or two, but only through prolonged actions over 50 years and perhaps longer. In addition, the oceans would continue to warm for decades even if emissions were halted. Ecological changes due to climate change that has already occurred will continue to unfold for decades. CO2 resides in the atmosphere and surface ocean for centuries and is only slowly taken up by the deep ocean. If we were to reduce our emissions to zero immediately, it would take more than 200 years for terrestrial and oceanic uptake of carbon to restore the atmosphere to its pre-industrial condition. Thus, there is great momentum in the climate system, in the heat capacity of the oceans, in ice sheets, and in the residence time of carbon dioxide in the atmosphere, and this fact makes a certain amount of climate change inevitable. Future impacts discussed in Unit 12, "Earth's Changing Climate," include sea level rise, changes in rainfall patterns, early melting of mountain snow pack and glaciers (which serve as the primary water supply for billions of people), changes in storms and tropical cyclones, and other ecological changes that affect ecosystems crucial to human society.

One source of confusion in discussions on how to reduce CO2 emissions is that our energy system is really more than one system. As discussed in Unit 10, "Energy Challenges," we use energy for transportation, for electricity to power our lights and electronics, to heat or cool our homes, for manufacturing, and for agriculture. Our energy choices within each of these sectors come with different technological constraints that require different types of solutions if reductions in CO2 emissions are to be achieved. For example, the internal combustion engine (along with the gas turbine in airplanes) currently dominates the transportation sector and is fueled almost exclusively by petroleum, making transportation responsible for approximately 40 percent of global CO2 emissions. The electricity industry has a much broader set of energy sources, including coal, natural gas, nuclear, hydroelectric, wind, solar, biomass, and geothermal—although coal, natural gas, and nuclear are currently dominant. Thus, the discussion of strategies for mitigating climate change must address not just sources of energy, but sources in relationship to different societal needs (Fig. 4).

Another consideration is differences in energy technologies among countries. Some countries, such as Saudi Arabia and Russia, are rich in hydrocarbon resources, and this guides their energy decisions. Other countries, such as Japan, have almost no domestic energy resources and turn to technological solutions such as solar and nuclear power. The end use of energy also varies among countries. For example, both China and the United States have large coal reserves, but China consumes almost twice as much, in part due to the large manufacturing industry in China versus the service economy of the U.S. (Fig. 5). This means there can be no single strategy for how the world will address climate change but rather we need a portfolio of strategies. Rapidly developing countries will have different solutions from those of developed countries. Even countries with similar levels of economic development will employ different solutions because of geography, political and cultural attitudes, and political systems.

This does not mean that individual solutions must be created for all countries. CO2 emissions are not distributed evenly; a handful of countries contribute most of the emissions and will be responsible for bringing about most of the reductions. If the United States, China, India, the European Union, Russia, Japan, Australia, Canada, and perhaps Indonesia and Brazil each take significant steps to reduce emissions, it is likely that such efforts will be successful in reducing the impacts of climate change on the rest of the world (Fig. 5).

Another constraint is the timescale over which it is possible to build new energy systems. Eliminating carbon emissions from electricity generation by using nuclear power, for example, would require building two large nuclear plants each week for the next 100 years. This rate of change is simply not possible given current constraints on steel production, construction capacity, the education of operators, and many other practical considerations. Taken together with the diverse uses of energy and the different needs of different nations, this means that there is no silver bullet solution for the climate–energy challenge. Myriad approaches are required. One can group these approaches into three broad categories, each of which will play an essential part in any serious climate mitigation effort.

Reduction of Energy Demand

The first category involves reducing CO2 emissions by reducing energy consumption, as discussed in section 2, "Measuring (and Reducing) the Human Footprint." This does not necessarily require reducing economic activity, i.e. consuming less (although this can be part of the solution); rather, it means restructuring society, either by investing in low-energy adaptations such as efficient public transportation systems or by adopting energy-efficient technologies in buildings, in automobiles, and throughout the economy.

Huge discrepancies in energy efficiency exist today among developed countries. In general, countries with higher historical energy prices, such as most of Western Europe, are more efficient than countries with inexpensive energy including petroleum, although the differences can also be explained by historical investments in cities and suburbs and in highways and public transportation systems, as well as by a variety of other factors. But whatever the cause of the current differences among countries, there is great potential across the developed and the developing world to dramatically lower energy use through smarter and better energy systems.

Much of the efficiency gains can be accomplished with existing technologies, such as compact-fluorescent lighting or more efficient building designs; these are often referred to as the "low hanging fruit," as they are often economically advantageous because they are simple and inexpensive. In addition, there are technological improvements in end use that would contribute greatly to any emissions reductions effort by making large jumps in energy efficiency. For example, if we can develop batteries for electric automobiles that are economical and reliable, and if their use is broadly adopted, we will be able to replace the low-efficiency internal combustion engine with the high-efficiency electric motor. Moreover, electric cars would break the monopoly that petroleum currently has as the source of energy for transportation, thus addressing security concerns involving the geopolitics of oil and allowing transportation fuel to come from carbon-free sources. Whether better batteries are technically possible remains a question.

Non-Fossil Energy Systems

The second category of solutions to the climate–energy challenge involves expansion of non-fossil energy systems, including wind, solar, biomass, geothermal, and nuclear power. Again, there is no silver bullet. Wind is currently the most economical of these energy systems for electricity generation. However, wind requires huge excess capacity because of problems with intermittency, and so it cannot become a source for base load power unless storage technologies improve. Solar-generated electricity has similar problems with energy storage and is also expensive compared with wind or nuclear power. Nuclear power can be used for base load power, unlike wind or solar, but issues of safety, storage, and handling of nuclear waste and security concerns about nuclear weapons proliferation will have to be addressed before widespread expansion is likely, at least in the United States and Western Europe (aside from France, which already has made a significant commitment to nuclear energy).

This category is one with great hopes for technological "breakthroughs"—such as fusion, inexpensive solar, and inexpensive fuel cells—that may revolutionize our energy systems. Thus, basic research and development must be a part of any climate mitigation strategy. However, no responsible strategy should rely exclusively on breakthrough technologies; they may not exist for decades, if ever.

Outside of the electric realm, biomass converted to biofuel may play a major role in reducing CO2 emissions in the transportation sector, at least until powerful, inexpensive, and reliable battery technologies or some alternative transportation technologies are developed. For example, Brazil currently obtains most of its transportation fuel from fermentation of sugar cane into ethanol, and similar programs are being implemented around the world.

A more efficient technology may be the conversion of biomass into synthetic diesel fuel via the Fischer-Tropsch process, which was used by the Germans in World War II to transform coal into liquid fuel. This process has the advantage of creating a more diverse range of fuel products, including jet fuel for air transport, and of being more efficient through use of all types of biomass, not just sugar (or cellulose for a cellulosic conversion process). The Fischer-Tropsch process, which involves gasification of the biomass by heating it in the presence of oxygen, produces carbon monoxide and hydrogen. This "syngas" is then converted to liquid fuel by passing the gas over a cobalt or iron catalyst.

Carbon Sequestration

The third category of solutions involves CO2 capture from emissions sources and storage in geologic repositories, a process often referred to as carbon sequestration. This is an essential component of any climate mitigation portfolio because of the abundance of inexpensive coal in the largest economies of the world. Even with huge improvements in efficiency and increases in nuclear, solar, wind, and biomass power, the world is likely to depend heavily on coal, especially the five countries that hold 75 percent of the world's reserves: the United States, Russia, China, India, and Australia. However, as a technological strategy, carbon capture and storage (CCS) need not apply only to coal; any point source of CO2 can be sequestered, including biomass gasification, which would result in negative emissions.

The scientific questions about CCS deal with the reliability of storage of vast quantities of CO2 in underground repositories—and the quantities are indeed vast. Reservoir capacity required over the next century is conservatively estimated at one trillion tons of CO2, and it may exceed twice this quantity. This amount far exceeds the capacity of old oil and gas fields, which will be among the first targets for sequestration projects because of additional revenues earned from enhanced oil recovery. However, there is more than enough capacity in deep saline aquifers to store centuries of emissions, and also in deep-sea sediments, which may provide leakproof storage in coastal sites. In general, the storage issues do not involve large technological innovations, but rather improved understanding of the behavior of CO2 at high pressure in natural geologic formations that contain fractures and faults. Geologic storage does not have to last forever—only long enough to allow the natural carbon cycle to reduce the atmospheric CO2 to near pre-industrial levels. This means that storage for 2000 years is long enough if deep-ocean mixing is not impeded significantly by stratification. It seems likely that many geological settings will provide adequate storage, but the data to demonstrate this over millennia do not yet exist. A more expansive program aimed at monitoring underground CO2 injections in a wide variety of geologic settings is essential if CCS is to be adopted before the middle of the century.

The technological advances in CCS necessitate improving the efficiency of the capture of CO2 from a coal-fired power plant. Capture can take place either by postcombustion adsorption, or through design of a power plant (either oxy-combustion or gasification) that produces a pure stream of CO2 as an effluent. Either way, the capture of CO2 is expensive, both financially and energetically. It has been suggested that capture and storage combined would use roughly 30 percent of the energy from the coal combustion in the first place and may raise the cost of generating electricity from coal by 50 percent, with two-thirds of this increase coming from capture. Even though these estimates are uncertain, given that carbon sequestration is not yet practiced at any coal plant, it is clear that technological innovation in the capture of CO2 from a mixed gas stream is important.

Carbon sequestration also occurs through enhanced biological uptake such as reforestation or fertilization of marine phytoplankton. These approaches could be considered a separate category, as, for example, planting trees is quite different from injecting vast quantities of CO2 underground. If pursued aggressively, such strategies might offset CO2 emissions by as much as 7 Gigatons (Gt) of CO2 (2 Gt of carbon) per year by the end of the century, out of total emissions of more than 80 Gt per year of CO2 (22 Gt of carbon) as forecast in most business-as-usual scenarios. These approaches might be an important piece of a solution, but they will not replace the need for improved energy efficiency, non-fossil energy sources, and carbon sequestration.

WILL IT BE ENOUGH?

The nature of the climate experiment means that no one truly knows what a safe level of CO2 really is, apart from the impossible goal of the pre-industrial level of 280 parts per million (ppm). It is possible that nations will implement many of the approaches outlined above over the next few decades, which would stabilize atmospheric CO2 below 600 ppm; it is difficult to imagine that a much lower stabilization level will be realized given the current state of the world energy systems. It is possible that this effort will be enough, that the world will warm another 2 or 3°C, that ice sheets will slowly melt, and that most of the severe consequences will be gradual, allowing adaptation by humans and natural ecosystems. On the other hand, it is also possible that even with concerted effort and cooperation among the large nations of the world, the climate system will respond too quickly for humans to adapt, that the Greenland and West Antarctic ice sheets will decay more quickly than expected, and that the impacts of a warmer world on humans and on natural ecosystems will be worse than we now predict.

It is very difficult to know which scenario is correct. The magnitude of the consequences depends in part on how we deal with them. Because of the potential for catastrophe, it seems prudent to ask what societies might do if the rate of climate change were to accelerate over the next few decades and if the consequences were to be much worse than anticipated. One approach that has long been discussed is the engineering of our climate system by adjusting the incoming solar radiation by means of reflectors in space or in the upper atmosphere; indeed, there may be some ways to accomplish a reduction in solar radiation at very low cost relative to other strategies of mitigation. Recently, such ideas have gained more prominence, not as a substitute for serious emissions reductions, but in the sober realization that efforts to reduce emissions may not be sufficient to avoid dangerous consequences. The power to engineer the climate comes with an awesome responsibility. How could we engineer such a system to be failsafe? Which countries would control this effort? Who would decide how much to use, or when? And what would happen if something went wrong, if we discovered some unforeseen consequences that required shutting the effort down once human societies and natural ecosystems depended on it?

Ultimately, our path in dealing with climate change, as with many other environmental challenges, will depend on the choices we make—not just we as individuals, but nations and human society as a whole. The good news is that there are strategies that can solve these problems. Tropical rain forests can be protected from deforestation. Marine ecosystems can be protected from overfishing. And although we are already committed to substantial climate change, we can choose to rebuild our energy infrastructure to avoid the worst impacts. Some of these choices involve new technologies that require spending money; others simply involve a change in behavior, perhaps enforced by laws and regulations. Environmental science helps clarify what the consequences of our choices are likely to be and hopefully guides society to make better choices in caring for our habitable planet.